1. Field of the Invention
This invention relates to a sealing device provided on the outer rim of the floating roof of an oil tank.
2. Description of the Prior Art
A sealing device, interposed between the inside of the peripheral wall of a floating roof type oil tank and the outside of the outer rim of said floating roof, is broadly classified into two types. One type is the so-called soft seal which is constructed by wrapping an elastic-body prepared from, for example, urethane foam, in a liquid-proof oil-resistant covering formed of, for example, nitrile butadiene rubber (NBR). The other type is the so-called mechanical seal which is constructed by closely attaching a slidable plate (referred to as "a seal shoe") to the surface of the peripheral wall of an oil tank by a mechanical force, and covering an interstice, defined between the floating roof and seal shoe, with a flexible material. The former type has been designed in anticipation of the occurrence of a fire which might arise due to a collision between the seal device and the peripheral tank wall when the oil level is made to sway vigorously as, for example, during an earthquake. However, said former type of sealing device is accompanied with drawbacks in respect to the wear resistance or oil-resistance of a sealing material such as NBR and the durability of an elastic material such as urethane foam, which might be reduced by compression creeping. Therefore, a seal used in an earthquake-free district mainly consists of the latter mechanical type.
FIGS. 1 and 2 illustrate the different but widely accepted techniques of the mechanical seal, including the peripheral wall 1 of a tank, a seal shoe 2, a connector 3' of the seal shoe 2 and a floating pontoon 5. Also shown is a seal 4'. The space between the pontoon 5 and tank wall 1 varies within a certain range due to errors in the roundness of a manufactured peripheral tank wall, and the vertical movement of a floating roof, caused by the efflux or influx of storage liquid. Therefore, the seal shoe 2 always has to be tightly attached to the tank wall 1 in conformity to the aforementioned change in the space between said pontoon 5 and tank wall 1. The mechanical seal techniques, shown in FIG. 1 and 2, differ from each other in the process of applying pressure in order to attach the seal shoe 2 tightly to the peripheral tank wall 1. FIG. 1 illustrates the process of applying said pressure by means of a plate spring 3". FIG. 2 indicates the process of applying said pressure by utilizing a weight 6 as a counterweight.
Throughout FIGS. 1 and 2, however, the seal 4' of the sealing device is so designed as to connect the upper portion of the seal shoe 2 and that of the floating roof 5. Therefore, a free level of stored oil is always present outside the peripheral wall of the floating roof 5. This condition has departed far from the fundamental object of an oil tank, which is minimizing the evaporation of stored oil to the best possible extent. The aforementioned seal 4' which in the past has been formed of NBR, for example, raises problems in respect to weatherproofness, for instance, the prevention of rain seepage.